First of all, the synopsis sounds good. Sure, it will be a dumb action flick and hardly reach the quality of a TNG Picard vs. Starfleet story (precisely because this movie will be a "one rotten apple" and not a "holy sh*t, there is something wrong with every second Admiral and thus our organization itself" story) but at least they do not fight against some oh-so-wicked looking Tattoomulans this time but against one of themselves. I guess that the story will be better than that of STXI which is admittedly hardly a dangerous bet.
Second, we have already known that the character is canonical, a villain and iconic; now we know in addition that he is a Starfleet officer whom Kirk personally knows. Mitchell is the obvious guess (in addition there is Urban's slip of the tongue about Mitchell as pro plus his seeming death in a comic as con) as Finnegan is hardly more than a bully or iconic ... and as you simply do not choose an Irishman as villain.
Obviously Mitchel is a far better choice than Khan as his character is far more flexible. He is just an arbitrary Starfleet officer with an overdose of self-confidence and a friend of Kirk whereas Khan is a megalomaniac superhuman who isn't all that interesting in and of himself and mainly works as personification of the fictional historical past.